The present invention relates to a process for the epoxidation of olefins by means of hydrogen peroxide and a titanium zeolite catalyst.
Prior Art
It is known from EP-A 100 119 that propene can be reacted with hydrogen peroxide to form propene oxide when a titanium-containing zeolite is used as catalyst.
However, the catalysts have the disadvantage that they steadily lose their catalytic activity during the reaction. For the epoxidation of propene, this is described in M. G. Clerici, G. Bellussi and U. Romano, J. Catal. 129 (1991) 159-167. For this reason, methods of maintaining the catalytic activity of the catalyst by periodic regeneration of the catalyst have been developed.
WO 99/01445 relates to a process for the epoxidation of olefins by means of hydrogen peroxide in which progressive deactivation of the catalyst is countered by increasing the pressure and temperature in order to maintain a predetermined minimum conversion for as long as possible and thus to prolong the interval between two catalyst regeneration cycles. However, increasing the pressure and temperature is subject to technical limits. Thus, secondary reactions occur to an increased extent when a maximum temperature is exceeded, so that, for example, the temperature cannot be increased at will. Even when the time between two regeneration cycles can be increased, separate regeneration of the catalyst is still absolutely necessary.
It is known from Clerici et al. that the catalyst can be regenerated by calcination at 550xc2x0 C. EP-A 743 094 describes a process for regeneration by calcination at a temperature of from 150 to 400xc2x0 C. in the presence of molecular oxygen. Furthermore, EP-A 790 075 describes a process for regeneration in a gas stream at a temperature of from 150 to 200xc2x0 C. in the absence of molecular oxygen. In all these processes, the regeneration is carried out by means of a gas phase and, owing to the high temperatures required, the catalyst generally has to be removed from the reactor used for the epoxidation, which is associated with additional costs.
It is also known from Clerici et al. that the catalyst can be regenerated by washing with a solvent at elevated temperature. However, in practice this process requires either very long times or significantly higher temperatures and can therefore not be implemented economically in industrial plants.
It is known from EP-A 757 044 that the catalyst can be regenerated by treatment with hydrogen peroxide in the absence of an olefin.
DE-A 198 05 552 teaches that this process can also be carried out with the catalyst remaining in the reactor used for the epoxidation during the regeneration. However, the process has the disadvantage that the epoxidation reaction has to be interrupted for the regeneration of the catalyst.
WO 98/18555 describes a process for regenerating a titanium zeolite catalyst which is used in the epoxidation of olefins by means of hydrogen peroxide. According to one embodiment, a solution of the oxidant which is also used for the epoxidation is employed as regeneration medium. For example, the reaction medium leaving the epoxidation reactor can, if appropriate after addition of hydrogen peroxide, be used as regeneration medium. This is largely free of unreacted olefins.
It is known from U.S. Pat. No. 5,849,937 that interruption-free operation of the epoxidation can be achieved when the reaction is carried out in a number of fixed-bed reactors connected in series and when the activity of the catalyst has dropped in one reactor, this reactor is taken out of production and replaced by a reactor containing regenerated catalyst. This procedure has the disadvantage that one more reactor and the corresponding amount of catalyst than is necessary for carrying out the epoxidation has to be kept at the ready.
All known regeneration processes have the disadvantage that the catalyst displays a step increase in activity after the regeneration and leads to increased formation of by-products formed by subsequent reactions of propylene oxide. This results in decreased yields and in problems in the operation of a continuous production plant due to the fluctuations in heat evolution and the concentrations of by-products.
It is therefore an object of the present invention to provide a process for the catalytic epoxidation of olefins by means of hydrogen peroxide and a titanium zeolite catalyst, in which process the catalytic activity of the catalyst is periodically regenerated without step changes occurring as a result of increased activity and by-product formation after the regeneration.
The above and other objects of the invention can be achieved by a process for the catalytic epoxidation of olefins by means of hydrogen peroxide and a titanium zeolite catalyst, wherein the epoxidation reaction is carried out in a reaction system through which the reaction mixture flows continuously and the regeneration of deactivated catalyst is carried out by means of hydrogen peroxide in the presence of the olefin while continuing the epoxidation reaction.
It has surprisingly been found that hydrogen peroxide is able, even in admixture with the starting olefin without interruption of the epoxidation reaction, to regenerate the deactivated catalyst. The result of the regeneration is better, the greater the concentration of the not yet reacted hydrogen peroxide in the reaction medium which comes into contact with the catalyst to be regenerated. Thus, it is particularly advantageous for the deactivated catalyst to be located close to the point at which hydrogen peroxide enters the reaction system for the regeneration.
A particularly preferred embodiment of the present invention provides a process for the catalytic epoxidation of olefins by means of hydrogen peroxide in the presence of a titanium zeolite catalyst in a reaction system through which the reaction mixture flows continuously, in which process the deactivated catalyst from the region in which the reaction product leaves the reaction system is placed near the point at which hydrogen peroxide enters, without the reaction being interrupted.